These are Duty Officer assignments, rather than Missions and Episodes, so you are likely to get an answer if you post in the Duty Officer System forum.
I'll tell you what I know, though.
Every exploration cluster and nebula offers a colonisation chain. This, as you stated, starts with "Colony site survey in the $cluster_or_nebula_name". Each chain has 7 parts.
It's a matter of luck and cooldown to get the extra parts. Check in each cluster after every sector assignment reset and you will find them. You also need to check the "Department Heads/Operations" tab, since that can sometimes contain colonial chain assignments.
Note that you only want to do the missions highlighted in yellow with an "X/7" note telling you which part of the chain it is - there are lots of similar sounding missions that are nothing to do with the chains.